The present invention relates to a solid-state imaging apparatus, and more particularly, to a solid-state imaging apparatus that performs exposure control and white balance control.
FIG. 1 is a block diagram showing the configuration of a conventional imaging apparatus 100 which uses a CCD image sensor, or CCD 1.
The CCD 1, which is a solid-state imaging device, has a plurality of light-receiving pixels, a plurality of vertical shift registers, and usually a horizontal shift register. The light-receiving pixels are arranged in a matrix form on a light-receiving surface at regular intervals and produce and store information charges corresponding to the image of a sensed object. The vertical shift registers are arranged to correspond to the columns of the light-receiving pixels and sequentially shift the information charges stored in the light-receiving pixels in the vertical direction. The horizontal shift register is arranged on the output side of the vertical shift registers and receives the information charges output from the vertical shift registers, and then transfers the information charges row by row. This allows the horizontal shift register to output an image signal A1 in accordance with the information charges stored in the light-receiving pixels.
When performing color image sensing, a color filter for color distribution is attached to the light-receiving surface in order to associate the individual light-receiving pixels of the CCD 1 with predetermined color components. There are stripe type and mosaic type color filters. Although the structure of the mosaic type color filter is more complicated than the stripe type color filter, the mosaic type filter has higher horizontal resolution. Thus, imaging apparatuses that require high resolution, such as a video camera, use mosaic type color filters.
A drive circuit 2 responds to various timing signals from a timing control circuit 3 and supplies a multi-phase drive clock to the shift registers of the CCD 1. For example, a 4-phase vertical transfer clock "PHgr"v is supplied to the vertical shift registers, and a 2-phase horizontal transfer clock "PHgr"h is supplied to the horizontal shift register. In accordance with a reference clock having a predetermined cycle, the timing control circuit 3 produces a vertical timing signal VD, which determines the vertical scan timing of the CCD 1, and a horizontal timing signal HD, which determines the horizontal scan timing, and supplies the timing signals to the drive circuit 2.
An analog processing circuit 4 performs a process, such as sampling and holding or level clamping, on the image signal A1 received from the CCD 1 to produce an image signal A2 which conforms to a predetermined format. For example, in the sample and hold process, only signal levels are extracted from the image signal A1, which has reset levels and signal levels alternately repeated in synchronism with the output operation of the CCD 1. In the level clamping process, a black reference level set at the end of the horizontal scanning period of the image signal A1 is clamped to a predetermined level every horizontal scanning period. An A/D converter circuit 5 quantizes the image signal A2 received from the analog processing circuit 4 to generate image data D3, which represents the information corresponding to each light-receiving pixel of the CCD 1 with a digital value.
A color computation circuit 6 receives the image data D3 from the A/D converter circuit 5, separates the data D3 into three color components, and generates color component data. The color computation circuit 6 further generates color data C4 corresponding to the three primary colors (R: red, G: green and B: blue) of light. For example, if the color filter has yellow (YE), cyan (Cy), green (G), and white (W) segments, the color component data C[Ye], C[Cy], C[G], and C[W] undergo color computation processes in accordance with the equations listed below to generate color data C4, which corresponds to the three primary colors of light.
Yexe2x88x92G=R
Cyxe2x88x92G=B
G=G
A white balance circuit 7 assigns specific gains to each of the color components in order to adjust the balance of each color component and generate adjusted color data C5. In other words, the white balance circuit 7 compensates for differences in the sensitivities of the light-receiving pixels of the CCD 1 that depend on each color component and individually sets the gain of each color component to improve the color reproduction of a reproduced image.
A color difference computation circuit 8 performs a computation process on the adjusted color data C5 received from the white balance circuit 7 and generates color difference data U and V. The color difference computation circuit 8 combines the R, G, and B components of the adjusted color data at a ratio of 3:6:1 to generate luminance data. Then, the color difference computation circuit 8 subtracts the luminance data from the B component to generate the color difference data U, and the luminance data from the R component to generate the color difference data V.
A luminance computation circuit 9 combines the plurality of color components (in this case, four) included in the image data D3 to generate luminance data B4. That is, if the components Ye, Cy, G, W are combined, the following equation is obtained.                               Ye          +          Cy          +          G          +          W                =                              (                          B              +              G                        )                    +                      (                          R              +              G                        )                    +          G          +                      (                          R              +              G              +              B                        )                                                  =                              2            ⁢            R                    +                      4            ⁢            G                    +                      2            ⁢            B                              
This generates luminance data in which the R, G and B components are combined at a ratio of 1:2:1. While the NTSC standards define a luminance signal produced by combining the R, G and B components at a ratio of 3:6:1, a luminance signal produced by combining the components at a ratio close to this ratio does not cause a practical problem.
An outline correction circuit 10 emphasizes a specific frequency component included in the luminance data B4 to generate aperture data and adds the aperture data to the luminance data B4. In other words, to emphasize the image outline of a sensed object, the outline correction circuit 10 performs a filtering process on the luminance data B4 to emphasize a frequency component that is one fourth the sampling frequency of the image signal D3 output by the A/D converter circuit 5 and generate the aperture data. The luminance data B4 generated by adding the aperture data is provided as luminance data Y to an external display device or recording device together with the color difference data U and V.
The solid-state imaging apparatus 100 determines the exposure state based on the level of the image signal and feeds back the determination result to the timing control circuit 3. The timing control circuit 3 decreases and lengthens the exposure time of the CCD 1 based on the determination result to obtain an appropriate exposure time. The exposure time of the CCD 1 is the period between when the storing of the information charges starts to when the transmission of the information charges starts. Therefore, an appropriate amount of information charges may be stored in each light-receiving pixel by changing the time point for starting the storing of the information charges. Further, the gain of each color component is determined based on the average level of the image signal, and the determined gain is applied to the color component to perform white balance control.
During exposure control of the CCD 1, the exposure state is determined during each vertical scan period, and the exposure time of the CCD 1, or the shutter timing, is updated every vertical scan period. This enables the CCD 1 to follow changes in the luminance of the sensed object. In comparison, during white balance control, the color balance of the sensed object changes more gradually than the luminance of the sensed object. Thus, the gain set for each color component is updated in cycles that are longer than that of the exposure control.
When the imaging apparatus 100 is operated under a light source that emits light in a cyclic manner, flicker of the reproduced image does not occur theoretically as long as the imaging cycle and the light emission cycle of the light source is the same or have a relationship that can be obtained by multiplying an integer. However, if jitter is included in the cyclic light emission of the light source, a slight difference may be produced between the imaging cycle and the light emission cycle. This causes the level of the image signal to fluctuate. In such state, the exposure control conditions are updated during each vertical scan period. Thus, exposure control is substantially not affected by jitter. However, the white balance control has a higher possibility of being affected by jitter since the response of the white balance control is slower than that of the exposure control. Hence, when the exposure time of the CCD 1 becomes short, fluctuations of the signal level caused by jitter increase. This changes the color of the reproduced image in cycles determined by the jitter included in the light emission of the light source.
It is an object of the present invention to provide a solid-state imaging apparatus that performs exposure control and white balance control in a stable manner.
To achieve the above object, the present invention provides a solid-state imaging apparatus including a solid-state imaging device for accumulating information charges corresponding to an image of a sensed object and generating an image signal consisting of a plurality of color components. A drive circuit is connected to the solid-state imaging device to drive the solid-state imaging device so that the image signal is generated within a predetermined time period. A timing control circuit is connected to the drive circuit to provide a timing signal to the drive circuit. The timing signal determines a length of the predetermined time period. An exposure control circuit is connected to the imaging device and the timing control circuit to determine an exposure state of the imaging device based on the image signal in a first cycle and control the timing control circuit in order to shorten or lengthen the predetermined time period. A white balance circuit is connected to the imaging device and the exposure control circuit to perform a predetermined process on the plurality of color components of the image signal in a second cycle, which is longer than the first cycle, so that the plurality of color components are relatively balanced with one another. The white balance circuit performs the predetermined process in a third cycle that is shorter than the second cycle when the exposure control circuit determines that the exposure state is stable.
Another aspect of the present invention provides an imaging apparatus including an image sensor for capturing an image of a sensed object and generating an image signal thereof. An exposure control circuit receives the image signal, compares the image signal to predetermined upper and lower limit values, and calculates a control signal based on the comparison results. A timing control circuit is connected to the exposure control circuit and receives the control signal. The timing control circuit generates a plurality of timing signals for specifying a vertical scan period and a horizontal scan period of the image sensor based on the control signal. A balance control circuit is connected to the exposure control circuit to receives the control signal, generates gain information and provides the gain information to an image processing circuit of the imaging apparatus, so that a change in the exposure time caused by the control signal is matched with a corresponding change in gain information used to process the image signal.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.